Shuumatsu Nani Shitemasu ka? Isogashii Desu ka? Sukutte Moratte Ii Desu ka? (Novel)

Synopsis

From Yen On:
Everyone wants to be remembered. Untold years after the strange Beasts drove humanity to extinction, Willem emerges from his slumber as the sole survivor. A new civilization of various other races has risen on the floating islands, but there's no place for a featureless human among them. After aimlessly wandering for a time, Willem grudgingly accepts a job with the military, only to discover that the "weapons" he watches over are in fact young faeries. As he slowly learns more about the children and how they're linked to his past, the war with the Beasts grows increasingly desperate. When the girls prepare to fight a hopeless battle, is there anything a lone human can do?

Synopsis & details from MangaUpdates